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SubjectRe: [PATCH] KVM: x86: Convert tsc_write_lock to raw_spinlock
On 02/07/2011 07:23 PM, Jan Kiszka wrote:
> On 2011-02-07 18:10, Avi Kivity wrote:
> > On 02/07/2011 06:59 PM, Jan Kiszka wrote:
> >>>
> >>> (well, actually, cpufreq_notifier and kvm_arch_hardware_enable are
> >>> already non preemptible, and the stats code should just go away?)
> >>
> >> The stats code is trivial to convert, so it doesn't matter.
> >
> > Removal is easier.
>
> Is that stat interface no longer used?

It's there for compatibility. I'm itching to remove it. See
qemu-kvm.git/kvm/kvm_stat for the only known user, and for the
replacement via tracepoints.

Tracepoints have marginally lower overhead when disabled, and somewhat
higher overhead when enabled. A disadvantage of tracepoints is that it
is harder to associate an event with a vm when that event is triggered
by a workqueue, but I don't think it matters in practice (kvm_stat
doesn't even provide a per-vm breakdown).

> >
> >> But what about mmu_shrink and its list_move_tail? How is this
> >> synchronized against kvm_destroy_vm - already today?
> >
> > kvm_destroy_vm() takes kvm_lock. If a vm is destroyed before
> > mmu_shrink(), mmu_shrink() will never see it. If we reach mmu_shrink()
> > before kvm_destroy_vm(), the latter will wait until mmu_shrink() is done.
> >
>
> Ah, I was confused. Would require some more logic if we wanted to make
> the loop lock-less, though.

Yes, the usual rcu_read_lock() / grab reference / rcu_read_unlock().

--
error compiling committee.c: too many arguments to function



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